In view of environmental problems and energy problems, fuel cells have attracted much attention as clean power-generating devices, because they generate electric power by a reverse reaction of electrolysis by using hydrogen and oxygen, and they produce no exhaust material other than water
The fuel cells can be classified into several kinds, depending on the kind of the electrolyte to be used therefor. Among such fuel cells, solid polymer electrolyte-type fuel cells can work at a low temperature, and therefore they are most useful for automobile or public or civilian uses. This type of fuel cell is constructed by stacking unit cells, each of which comprises, e.g., a polymer electrolyte, a gas diffusion electrode, a catalyst and a separator, and the fuel cell can attain high-output power generation.
In the fuel cell having the above structure, the separator for partitioning the unit cells usually has at least one flow channel (or groove) to which a fuel gas (such as hydrogen) and an oxidant gas (such as oxygen) are supplied, and from which the produced water content (steam) is discharged. Therefore, the separator is required to have a high gas impermeability capable of perfectly separating these gases, and is also required to have a high electroconductivity to reduce the internal resistance. Further, the separator is required to be excellent in heat conductivity, durability, strength, etc.
To satisfy these requirements, the separator has been heretofore studied in view of both aspects of metal and carbon materials to be used therefor. Among these materials, metals have a problem in the corrosion resistance thereof and therefore, an attempt has been made to cover the surface thereof with a noble metal or carbon. However, even in such a case, a sufficiently high durability cannot be obtained and moreover, the cost for covering the metal is problematic.
On the other hand, a large number of carbon materials have been studied as materials for constituting fuel cell separators, and examples thereof include a molded article obtained by press-molding an expanded graphite sheet, a molded article obtained by impregnating a carbon sintered body with a resin and curing (or hardening) the resin, a vitreous carbon obtained by baking a thermosetting resin, and a molded article obtained by mixing a carbon powder and a resin and molding the resultant mixture.
For example, Patent Document 1 discloses a complicated process such that a binder is added to a carbon powder and mixed under heating, the mixture is CIP (Cold Isostatic Pressing)-molded, baked and graphitized, and the thus obtained isotropic graphite material is impregnated with a thermosetting resin and subjected to a curing treatment, and grooves are engraved therein by cutting. Patent Document 2 discloses a technique of impregnating a paper containing carbon powder or carbon fiber with a thermosetting resin, stacking and press-bonding the resultant papers, and baking the stacked body. Patent Document 3 discloses a technique of injection-molding a phenol resin into a separator-shaped metal mold and baking the molded resin.
Materials subjected to a baking treatment as in these examples exhibit high electroconductivity, but the baking takes a long time and the productivity is low, and these materials have a certain problem such that they have a poor flexural strength. Further, when cut working is necessary, the mass productivity is low and the cost becomes higher, and therefore, these materials can be hardly popularized in future.
On the other hand, with respect to the electroconductivity of a fuel cell separator, it is important to reduce the contact resistance, which is a factor controlling the electroconductivity. There have been made some attempts to reduce the contact resistance. For example, Patent Document 4 discloses a method of covering the surface of a separator with metal or carbon having a high electroconductivity. Patent Document 5 discloses a method of covering the surface of a molded product of an electroconductive resin composition with an electroconductive polymer. Patent Document 6, discloses a method of covering the surface of a separator with an electroconductive material, or a method of embedding an electroconductive material in a separator so that the electroconductive material is disposed in the longitudinal direction of the separator.
On the other hand, Patent Document 7 discloses a method of increasing the area rate of carbon powder, by grinding the surface layer of a separator which is rich in a resin (i.e., a layer predominantly comprising a resin). Patent Document 8 discloses a method of improving the adhesion of the contact surface of a separator, by using a rubber as a binder therefor.
[Patent Document 1] JP-A (Japanese Unexamined Patent Publication) 8-222241
[Patent Document 2] JP-A 60-161144
[Patent Document 3] JP-A 2001-68128
[Patent Document 4] JP-A 2001-196076
[Patent Document 5] JP-A 2002-8685
[Patent Document 6] JP-A 2001-52721
[Patent Document 7] JP-A 2003-282084
[Patent Document 8] JP-A 2001-216977
As described above, the fuel cell separator is particularly required to have a high electroconductivity and a strong corrosion resistance, and to be produced at a low cost. In such a viewpoint, a carbon-based material mold-shaping type which is capable of omitting a cutting step attracts much attention, and the development thereof is promoted. However, in the case of such a carbon-based material, it is necessary to increase the amount of an electroconductivity-imparting material to be contained in the molded product in order to impart the molded product with a high electroconductivity. However, in such a case, the reduction in the amount of a resin to be contained in the molded product is limited to a certain extent, because the molding property should be retained. Accordingly, it is impossible to obtain a sufficiently high electroconductivity.
In addition, because of the large amount of the electroconductivity-imparting material contained in the molded product, the surface of the resultant molded product inevitably has a low smoothness, and a higher hardness, so that the contact resistance of the molded product produced from of an electroconductive resin composition tends to be deteriorated, and the resultant product becomes fragile. In addition, when the surface of the molded product is covered with the binder resin, and the contact resistance thereof is deteriorated, it is necessary to adopt a method of grinding the surface of the molded product.
Further, when the production process includes a baking step of heating the molded product at a high temperature of 1000-3000° C. for a long period so as to obtain a high electroconductivity, the time required for producing the molded product becomes longer, and further the production steps become complicated, and the production costs problematically become high.
On the other hand, a multilayer-type separator has a problem such that the adhesion in a boundary portion between the different kinds of layers is mainly problematic.